1. Field of the Invention
The present invention relates to an ionically conducting molecule and an ionic conductor which can be utilized as a solid conductor for a battery, and to a process for producing the ionic conductor.
2. Description of the Related Art
In general, since an ionically conducting polymer exhibits ease of processability into films, is lightweight and flexible, it is expected to find application in the field of electrochemistry, in particular in an all-solid lithium secondary battery.
There are several types of ionically conducting polymer. A single ion conducting polymer is a polymer in which only the cations act as charge carriers. A bi-ion conducting polymer is a polymer in which both the cations and the anions act as charge carriers. Single ion conducting polymers are preferable to bi-ion conductors for use in lithium secondary batteries for the following reasons. When a bi-ion conductor is used, the electrodes are blocking with respect to the anions. Thus, during charge and discharge processes, anions accumulate at the positive electrode which causes polarization of the conducting film. As a result of this phenomenon, the ionic conductivity decreases with time.
Therefore, in order to make an ionically conducting polymer into a single ion conductor, in which only the cations act as the charge carriers, it is necessary to fix the counter anion onto the ionically conducting polymer structure.
However, even in the aforementioned case, ion pairing of the cations with the fixed anions hinders movement of the former. Accordingly, mobility of the cation decreases and the ionic conductivity is greatly reduced.
The following methods have been thought of in order to reduce the influence of ion pairing. For example, the electron density on the anion is reduced by introduction of an electron withdrawing group; the cation is hindered stereochemically from approaching the anion by introduction of a bulky substituent group around the anion; and the distance between the fixed anions is shortened so as to reduce the activation energy barrier for cation motion.
For instance, Japanese Unexamined Patent Publication (KOKAI) No. 8-339,827 discloses the method for reducing charge density on the anion by introducing an electron withdrawing group. This idea is to fix the anion onto the polymer structure during polymer synthesis. Consequently, in order to synthesize the structure containing fixed anions, a complex synthetic route is necessary and the synthesis is difficult.
Japanese Unexamined Patent Publication (KOKAI) No. 10-223,258 discloses a non-aqueous lithium battery in which the ionic conductivity is improved by dissolution of a capacity attenuation inhibitor additive compound in the electrolyte. In this non-aqueous battery, the ionic conductivity is enhanced by using an organic solvent which takes part in the ionic conductivity and in which the capacity attenuation inhibitor additive compound is dissolved. Here, since the movement of the ions, which results in the ionic conductivity, is carried out in the organic solvent, the capacity attenuation inhibitor additive compound has a boroxine ring structure, which has a side chain composed of an alkyl group, so that it is likely to dissolve in the organic solvent.
In the lithium battery set forth in Japanese Unexamined Patent Publication (KOKAI) No. 10-223,258, the ionic conductivity is improved in the electrode member. However, only the conductivity of the electrolyte itself is upgraded. Hence, the ion conductivity of the ion conducting polymer is not enhanced.
It is therefore an object of the present invention to provide an ionically conducting molecule, an ionic conductor, which exhibits high mechanical strength and ionic conductivity, and a process for producing the same.
In order to carry out the object, the inventors of the present invention thought of preparing an ionically conducting molecule which both traps anions and provides an ion conducting pathway. They succeeded in obtaining an ionically conducting polymer which contains a boroxine ring for anion trapping and a molecular chain which provides an ion conducting pathway. Moreover, by using a structural member in which the tonically conducting molecule and salt are dispersed and immobilized, they successfully prepared an ionic conductor which is good in terms of mechanical strength and exhibits high ionic conductivity.
An ionically conducting molecule according to the present invention comprises:
a molecular chain which provides an ion conducting pathway; and
a boroxine ring bonded to the molecular chain and trapping an anion.
An ionic conductor according to the present invention comprises:
an electrolyte salt for ionic conduction;
an ionically conducting molecule including a molecular chain which provides an ion conducting pathway and a boroxine ring bonded to the molecular chain and trapping anions resulting from the electrolyte salt; and
a structural member for dispersion and immobilization of the ionically conducting molecule and the electrolyte salt.
A process for producing an ionic conductor according to the present invention comprises the steps of:
synthesis of an ionically conducting molecule including a molecular chain which provides an ion conducting pathway and a boroxine ring bonded to the aforementioned molecular chain and trapping anions; and
dispersion and immobilization of the ionically conducting molecule and an electrolyte salt in a structural member by compounding the ionically conducting molecule, the electrolyte salt and the structural member.
The ionically conducting molecule according to the present invention increases the fraction of charge carried by cations because the boroxine rings trap the anions of the electrolyte salt. The behaviour of the ionic conductor approaches more closely that of a single ion conductor. Note, by using boroxine rings, we do not get a single ion conductor but due to the strong interaction between the anions and the boroxine ring, the mobility of the anions is reduced and therefore more of the charge is carried by cations. Namely, the behaviour of the ionic conductor more closely approaches that of a single ion conductor. Moreover, since the anion is trapped by way of interaction of electrons on the anion with the boroxine ring, there is a much smaller contribution of these anion electrons towards ion pairing with the cation. As a result, the ionic conductivity due to the cation is improved. In addition, in the ionic conductor according to the present invention, the structural member improves the mechanical strength of the ionic conductor.
The process for producing the ionic conductor according to the present invention comprises the step of synthesizing the aforementioned ionically conducting molecule, and the step of dispersion and immobilization of the ionically conducting molecule and the electrolyte salt in a structural member by compounding the ionically conducting molecule, the electrolyte salt and the structural member, thereby producing the ionic conductor. Moreover, when an annealing step is carried out after the dispersion and immobilization step, the ionic conductivity of the ionic conductor is enhanced.
The ionically conducting molecule according to the present invention has a boroxine ring structure, which is an electron pair acceptor working as a receptacle (anion trap) for anions, and a molecular chain which provides an ion conducting pathway, and which promotes conduction of the cations. Accordingly, the boroxine ring structure interacts strongly with the anionic electron pair of the electrolyte salt in the ionic conductor, thereby reducing the mobility of the anions and causing the ionically conducting system to exhibit a high cation transport number. Therefore, a cation transport number enhancement is observed, and this transport number can more closely approach 1.
The ionic conductor according to the present invention has the aforementioned tonically conducting molecule and an electrolyte salt for carrying out ionic conduction in the structural member. The structural member improves the mechanical strength of the ionic conductor. Consequently, the ionic conductor makes a tough, solid ionic conductor. Moreover, the present ionic conductor exhibits a small interfacial resistance between itself and metallic lithium, and the small interfacial resistance is stable with time. Therefore, there are great expectations of the present ionic conductor as a material for lithium batteries.